InTech-On Efficiency of Arq and Harq Entities Interaction in Wimax Networks

7/31/2019 InTech-On Efficiency of Arq and Harq Entities Interaction in Wimax Networks

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On Efficiency of ARQ and HARQ EntitiesInteraction in WiMAX Networks

Zdenek Becvar and Pavel MachCzech Technical University in Prague, Faculty of Electrical Engineering,

Czech Republic

1. Introduction

During data exchange in wireless networks, an error in transmission can occur. Corrupteddata cannot be further processed without a correction. A technique based on eitherAutomatic Repeat reQuest (ARQ) or Forward Error Correction (FEC) is conventionally usedto repair erroneous data in wireless networks. The ARQ is backward mechanism that uses afeedback channel for the confirmation of error-free data delivery or to request aretransmission of corrupted data. This method can increase a network throughput if radiochannel conditions are getting worse (Sambale et al., 2008). On the other hand, the ARQmethod increases the delay of packets due to the retransmission of former unsuccessfullyreceived packets. The FEC can increase users data throughput over the channel with poorquality despite the fact that additional redundant bits are coded together with users data at

the transmitter side. The method combining both above mentioned methods is calledHybrid ARQ (HARQ). All three error correction mechanisms are implemented on physicaland/or Medium Access Control (MAC) layer.

The performance of ARQ defined in standard IEEE 802.16e (IEEE802.16e, 2006) depends onthe setting of several parameters such as size of user data carried in a frame, size of ARQblock, size of PDU (Protocol Data Unit), limit of retransmission timeout timer, or type ofpacket acknowledgement (Lee & Choi, 2008). Evaluation of the type of packetacknowledgment for different channel condition is presented in (Kang & Jang, 2008). Inother paper, the authors evaluate the ARQ performance for different ARQ parameters(Tykhomyrov et al., 2007). This work is later on enhanced by analysis of the impact of PDUsize on IEEE 802.16e networks performance while ARQ mechanism is used (Martikainen etal., 2008). Further, a comparison of ARQ and HARQ performance in IEEE 802.16 networks ispresented in (Sayenko et al., 2008). This paper also compares the amount of overheadgenerated by ARQ and HARQ. The optimal PDU size and MAC overhead due to thepackets retransmission is analyzed in (Hoymann, 2005). The authors in (Sengupta et al.,2005) propose to adjust the MAC PDU size depending on the channel state to achieve thebest ARQ performance. The paper is extended for analysis of a combination of errorcorrection techniques such as ARQ, FEC or MAC PDU aggregation on the VoIP speechquality (Sengupta et al., 2008). Authors proof the improvement of VoIP speech quality byusing these techniques. In (Chen & De Marca, 2008), the authors investigate an optimizationof ARQ parameter setting from the link throughput point of view.


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Advanced Transmission Techniques in WiMAX148

In conventional WiMAX network, the ARQ and HARQ work independently on each other(IEEE802.16e, 2006). In standalone ARQ process, a number of blocks received with errors

increases as the link quality between the transmitter and the receiver decreases. Thus, ifthe Block Error Rate (BLER) is more significant, the amount of retransmitted blocks is

higher as well. It can be assumed that if the channel quality is high, most of the blocks aretransferred without errors and the number of unsuccessfully received blocks is kept to

minimum. In such case, the transmission of positive acknowledgement (ACK) of correctly

delivered blocks appears more often than negative acknowledgement (NACK) ofcorrupted blocks. This assumption is considered in (Becvar & Bestak, 2011), where

authors propose to send only NACKs to significantly reduce signaling overheadintroduced by ARQ mechanism.

On the other hand, the HARQ is able to detect and correct the most of the radio channel

errors. However, due to the limitation of a number of retransmissions, some data may not

be delivered without errors if only HARQ is utilized. Consequently, these data have to be

retransmitted by ARQ process. The conventional ARQ has to acknowledge all data

independently on the result of HARQ procedure. In order to significantly reduce signaling

overhead, an interaction of both ARQ and HARQ methods should be utilized, see, e.g.,

(Maheshwari et al., 2008)).

The contributions of this chapter are as follows. Firstly, the results of improved ARQ scheme

according to (Becvar & Bestak, 2011) cooperating with HARQ is compared to the results

achieved by the conventional ARQ scheme with enabled and disabled cooperation between

both entities. Secondly, while only one hop communication is assumed when data are sent

only between a mobile station (MS) and a base station (BS) in (Becvar & Bestak, 2011), this

chapter analyzes the impact of relay stations (RS), defined in IEEE 802.16j (IEEE802.16j,2009), on the performance of individual methods. The extended simulations are performed

considering various setting of parameters. The amount of generated overhead is the metric

for the performance assessment.

The rest of this chapter is organized as follows. In the next section, the principle of ARQ and

HARQ used in WiMAX are described. In addition, the optimization of conventional ARQ

scheme according to (Becvar & Bestak, 2011) is presented in this section. In the section 3, the

overhead of HARQ algorithm in WiMAX networks is evaluated. The section 4 provides an

overview on simulation model and contemplates the parameters applied in simulator. The

section 5 presents the simulation results. Last section gives our conclusions.

2. ARQ and HAQR in WiMAX

This section provides overview on conventional ARQ and HARQ used in WiMAXnetworks. Further, the innovative ARQ proposed in (Becvar & Bestak, 2011) is alsodescribed to enable easy understanding of results presented in next sections.

2.1 Conventional ARQ

The principle of conventional ARQ method according to the IEEE 802.16e standard and the

structure of users information carried in the frame are depicted in Fig. 1.


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On Efficiency of ARQ and HARQ Entities Interaction in WiMAX Networks 149

Fig. 1. Principle of conventional ARQ

In WiMAX, each data burst generated either by the MS or the BS is segmented into PDUs.

These PDUs are further mapped into MAC frame. A PDU usually consists several blocks

Nblock, which number is given by following equation:

blocks

i ,ki ,k data

i ,kARQ blocks

SN

S

(1)

where i ,kdataS is a total size of data of i-th user in k-th frame, and similarlyi ,k

ARQ blocksS

represents a block size defined by parameter denoted in the standard as ARQ_Block_Size

(IEEE802.16e, 2006). This parameter is carried in TLV (Type/Length/Value) section of

registration messages (REG-REQ/RSP) exchanged between the BS and MS (see

(IEEE802.16e, 2006)). The parameter ARQ_Block_Size can take values from the following

range: 16, 32, 64, 128, 256, 512 and 1024 bytes. During a transmission, a sequence of

consecutive blocks is sent in the PDU. After that the receiver evaluates whether the data are

received correctly or not and sends an appropriate feedback message to the transmitter.

Note that all transmitted blocks (Nblock) have to be confirmed by ACK or NACK even if all

blocks are received without errors. The IEEE 802.16e standard defines four types of

acknowledgments: Selective ACK entry, Cumulative ACK entry, Cumulative with Selective

ACK entry and Cumulative with Block Sequence ACK entry.

The first type of acknowledgment uses selective maps to provide feedback to the

transmitter. In the selective map, each bit corresponds to one ARQ block. A bit set to 1

indicates error-free reception of the corresponding ARQ block. The second type, Cumulative

ACK entry, is based on the utilization of sequence maps. A sequence map defines a group of

consecutive blocks where each group includes a sequence of only erroneous blocks or

sequence of only error free blocks. The sequence maps can contain two or three sequences

with a length of 64 or 16 blocks respectively. The third type of ACK combines the previoustwo types. Finally, the last type combines the second type with ability to acknowledge ARQ

blocks in the form of block sequences.

The ACK or NACK is sent through above mentioned feedback message. The feedback is

transmitted in the next frame after the data transmission. The feedback message contains 8

bit field indicating Message ID and the rest of the message is dedicated to field consisting

ARQ_Feedback_Payload. The ARQ payload can be carried either via standalone ARQ

feedback message or by piggybacking the ARQ payload to the users data block. The

payload is always carried in a single PDU. The ARQ_Feedback_Payload includes one or

more ARQ_Feedback_IE (see Table 1) where IE stands for an Information Element.


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Syntax Size Notes

CID 16 bits Connection ID

Last 1 bit Identify the last IE in ARQ_Feedback

ACK Type 2 bits

0x0...Selective ACK

0x1...Cumulative ACK

0x2...Cumulative with Selective

0x3...Cumulative with Block Sequence

BSN 11 bits Block Sequence Number (0...2047)

Number of ACK Map 2 bits Number of Maps (M) = 1,2,3 or 4

MapsM x 16bits

Selective (16 blocks) or

Cumulative maps (2 x 64 blocks / 3 x 16 blocks)

Cumulative maps: 1 bit sequence format (2 or 3blocks), 2/3bits Sequence ACK (ACK/NACK ofsequence), (2x6) / (3x4) bits Sequence length

Table 1. Structure of ARQ_Feedback_IE (IEEE802.16e, 2006)

The size of an IE of each ARQ feedback message can be calculated according to equation:

ARQ_ FB_ IESize [bits] 32 M 16

(2)

where M represents the number of maps carried in one ARQ_Feedback_IE (see Table 1).Consequently, the overall size of whole feedback message is given by following formula:

IE

i

N

ARQ_ FB ARQ_ FB_ IEi 1

Size [bits] 8 Size

(3)

where NIE corresponds to the amount of information elements carried in one ARQ Feedback

message and the first eight bits represents the ARQ feedback message overhead (i.e.,

Message ID field). The overhead transmitted in all considered frames (Nframe) is equal to the

sum of partial overheads over the Nframe:

frame

i

N

ConvARQ ARQ _ FBi 1

OH [bits] Size

(4)

As indicated in Fig. 1, the retransmission of erroneous blocks cannot be accomplished beforethe third frame after the original transmission since the transmitter receives NACK in thenext frame after transmission (2nd frame). Hence a request for additional resources can becreated earliest at the upcomming frame (3rd frame). Therefore, the dedicated resources arenot available before the 4th frame. The retransmission of data (burst #2 in Fig. 1) can bescheduled either together with normally ordered data (burst #5 in Fig. 1) or the new data


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(burst #5 in Fig. 1) can be delayed by one frame. It causes a delay of retransmitted packetswith duration that corresponds to at least 3 times of frame duration (e.g., if the frameduration is 10 ms, the packet delay is at least 30 ms).

The new and retransmitted data are sent within the same frame only if the requestedcapacity (new data plus retransmitted data) is available. The WiMAX technologyimplements Stop-and-Wait mechanism that requests a confirmation of the previous blockbefore transmitting subsequent blocks. The number of blocks that can be unconfirmedbefore a transmission of the consequent blocks is defined in the standard by the parameterARQ_Window_Size.

2.2 Innovative ARQ

The innovative ARQ takes into consideration that the number of blocks received with errors

increases as the link quality between transmitter and receiver decreases. This scheme

adaptively selects one of three different ways of data delivery confirmation: i) conventionalARQ, ii) transmission of only NACK (ARQ Scheme I), iii) retransmission of only corrupted

blocks (ARQ Scheme II).

The first type of data acknowledgement, the conventional ARQ, was already explainedbefore.

The second type (ARQ Scheme I) assumes ARQ feedback message and ARQ_Feedback_IEswith the same structure as the conventional IEEE 802.16 ARQ feedback message. Howeverin this proposal, the ARQ feedback is sent only if a received PDU contains at least oneerroneous block. If all blocks in the PDU are error free, no feedback is sent. The PDU isassumed to be correctly transferred if the transmitter receives no feedback in the following

W frames after the transmission. If the feedback with NACK is not delivered, the dataconveying the delay sensitive services (e.g., VoIP) are assumed to be lost since the delaycaused by repeated ARQ retransmission is significant. In case of services not sensitive todelay, data belonging to lost NACK can be retransmitted using upper layer protocols, e.g.,TCP (Transmission Control Protocol). As the probability of lost packet or packet with errorstogether with the NACK feedback is very low, the increase of overhead due to upper layerprotocols is negligible.

The third way of data acknowledgement (ARQ Scheme II) is based on the same assumptionsas the previous one. The ACK feedback is likewise transmitted only if there is at least oneblock with errors. A block is assumed to be error-free if no feedback is received in one of the

following W frames after the transmission of appropriate data frame.

The overhead generated by the innovative ARQ (denoted as ARQ PIII) by a user in one

frame can be calculated according to the following equation:

IE

IE

N

ARQ_ FB_ III NSize 8 18 min 16 16 M ,10 B 11 res

(5)

where NIE is the number of IEs carried in one ARQ feedback message, MNIE corresponds tothe number of ACK maps in ARQ_Feedback_IE, B stands for the number of BSNs includedin one message and res is the number of bits used for an alignment of the feedback message


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length to integer number of bytes. The overhead generated by new ARQ scheme is given bythe following equitation:

frame

Nframe

N

SchemeIII ARQ _ FB_ III OH Size (6)

2.3 HARQ

The utilization of ARQ with support of FEC is known as HARQ. The HARQ method usesnot only retransmitted packets to reconstruct the original error free packets, but it alsoutilizes the packets received with errors. The original packet can be reconstructed by acombination of several versions of packet with errors. The HARQ described in (IEEE802.16e,2006) uses two different types of reconstruction: Chase Combining (CC) and IncrementalRedundancy (IR).

The first version of HARQ is denoted as Type I HARQ Chase Combining. In this case,blocks of data together with a CRC code are encoded using a FEC coder before transmission.If the channel quality is low and errors of data are identified, the data block is not discardedhowever it is kept in the memory. In the next phase, the receiver requests for retransmissionof this data block. The retransmitted block of data is then combined with the previous blocksreceived with errors. Combining more versions of the data blocks improves the probabilityof correct decoding even if all of them are received with errors.

Optionally, the IEEE802.16 standard also supports type II HARQ, which is known asIncremental Redundancy. In case of IR HARQ, the FEC coder codes one packet into severalsubpackets. Each of subpacket is coded with different code ratio. The subpackets are

distinguished by 2-bits SubPacket IDentifier (SPID). If the packet is transmitted for the firsttime, the subpacket with SPID=00 is sent. The successful receive of the packet at thedestination station is indicated by ACK. Otherwise, the transmitter sends a NACK and thetransmitter has to send another packet carrying one of four subpackets. Both received packet(the first transmission and retransmissions) are again combined by receiver to increase theprobability of correct decoding.

The overhead introduced by HARQ in WiMAX depends on the HARQ Type as follows.Firstly, the acknowledgment of HARQ bursts by modification of AI_SN (HARQ IdentifierSequence Number) of appropriate ACID (HARQ Channel ID) is assumed (for moreinformation, see (IEEE802.16e, 2006)). The AI_SN is included in HARQ DL or UL. The size

of HARQ map can be described by the subsequent formula:

DL

UL

64 SubB res ...RegionID_ONHARQOH

40 SubB res ...RegionID_OFF

48 SubB res ...RegionID_ONHARQOH

24 SubB res ...RegionID_OFF

(7)

where SubB is a size of management overhead according to a sub-burst. The amount ofmanagement overhead also depends on the utilization of Region ID (see (IEEE802.16e,


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2006)). In the simulations performed in this chapter, the Region ID is not considered. Theactual amount of bits of SubB depends on the HARQ Type. Based on the (IEEE802.16e,2006), the size of message according to the sub-bursts is following:

CC sub

IR CTC sub

IR CC sub

SubB 8 N (RCID 20 DIUC)SubB 8 N (RCID 20)

SubB 8 N (R 22 DIUC)

(8)

where Nsub is a number of sub-bursts; RCID represents a size of Reduced CID; and DIUC

represents the size of optional field, denoted as DIUC, containing 8 bits if included.

For the case when a low number of bursts are transmitted within a frame, the utilization of

so called Compact HARQ DL/UL maps enables to reduce an overhead (see (IEEE802.16e,

2006)). The overhead generated by compact version of maps is not dependent on the HARQ

type. The amount of overhead can be expressed by the next equations:

DL

UL

HARQOHcomp 12 RCID HCI CCI

HARQOHcomp 12 RCID HCI

(9)

where HCIis a size of HARQ control IE (8 bits if HARQ is enabled and 4 bits if HARQ is

temporary disabled); CCI is a size of CQICH control IE (16 bits if CQICH information are

included and 4 bits if the information are not included).

The simple evaluation of equations for full and compact HARQ maps enables to determine

which kind of maps generates minimum management overhead over the number of HARQ

sub-bursts (see Fig. 2). As the results show, the compact version of maps is profitable for all

numbers of sub-bursts in UL as well as for up to 12 sub-bursts in DL over all length of

Reduced CID.

(a) (b)

Fig. 2. Comparison of the overhead generated by compact and full HARQ maps for DL (a)and UL (b)


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The relation between BLER (for ARQ confirmation) and PER (for HARQ confirmation) isdefined according to (Provvedi et al., 2004) by following equation:

blocksNPER 1 (1 BLER) (10)

2.4 Cooperation of ARQ with HARQ to reduce signaling overhead

The mutual interaction consists in exchanging of information on successful packets

transmission between ARQ and HARQ entities (see Fig. 3). Therefore, the data confirmed by

HARQ need not to be confirmed again by ARQ process.

Fig. 3. Principle of ARQ and HARQ cooperation

At the side of transmitter, both ARQ and HARQ are implemented and applied on data.Similarly, the receiving side evaluates both ARQ and HARQ as well. However, ARQprocess on the receiving side need not to transmit all requests related to the corrupted dataif these data are already requested to be retransmitted by HARQ. The same way is appliedfor acknowledgement of data. In other words, data confirmed by HARQ are not further

confirmed by ARQ. The information on ACK/NACK is delivered to HARQ processes at theside of original transmitter and HARQ just provides information of ACK/NACK data to theARQ process at transmitter. Therefore, a part of overhead due to duplicated confirmation ofdata delivery is saved.

3. System model and simulation parameters

The simulator, developed in MATLAB, focuses on the evaluation of overhead generated by

ARQ and HARQ procedure in the uplink direction by one user (see Fig. 4). In simulations,

we assume direct communication between MS and BS as well as multihop communication

using RSs.


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Fig. 4. Link level simulation scenario

Each packet is transmitted either directly to the BS or over particular number of hops. Theprobability of block error between two stations is the same over all hops. Therefore, theoverall BLER of all hops (between the MS and the BS) is calculated according to thefollowing formula:

hopsN

MS BS hopBLER 1 BLER (11)

where BLERhop represents a BLER over each hop and Nhops is the number of overall hopsbetween the MS and the BS. Note that Nhops=n+1, where n is the number of RS in thecommunication chain.

If RSs are considered, the absolute level of transmitted overhead rises n times comparing tothe direct communication without RSs. This is due to the fact that feedback information istransmitted individually over each hop.

The setting of simulation parameters is depicted in Table 3. The evaluation is performed forBLER up to 10% per one hop. For higher BLER level, the channel is nearly unusable due tohigh error rate. Note that BLER of overall path from the MS to the BS is significantlyincreasing with rising number of hops (see (11)). The BLER of whole path from the MS to theBS is 27% if three hops are taken into account and if BLER of a hop is 10%.

For more precise evaluation, the overhead of upper layer is also considered. The TCPprotocol is assumed for an error correction by upper layer.

The user's data are transmitted in a number of frames transmitted from the BS to the MS.The overhead size is evaluated per all transmitted frames. A frame consists of one or severalPDUs and a PDU itself contains one or several ARQ blocks. The frames are subsequentlysent by the BS to the MS. A vector indicating positions of blocks with/without errors iscreated for each frame based on the given value of BLER. The MS responds to the BS bysending ARQ feedback message that includes selected ARQ scheme, ACK Type, and avector of errors in the transmission. According to the feedback message, the BS retransmitserroneous blocks as soon as possible, but not before the third frame after the originaltransmission. The size of users data in a DL frame is kept the same within each simulationdrop (1024 bytes or 4096 bytes).


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This process is repeated until all frames are sent to the MS and the MS confirms error-freereception of all blocks. The same vectors indicating positions of blocks with/without errorsare considered in all ARQ schemes.

Parameter Value

Number of frames 2000

BLER per hop [%] 0 10

Number of hops 1, 3

ARQ_Block_Size [bytes] 16 1024

PDU size [blocks] 1 16

ARQ ACK Types Selective, Cumulative

Max. HARQ retransmissions 2, 4

HARQ Type CC, IR-CTC

HARQ packet/burst size 1 PDU

RCID [bits] 7

Size of data in each DL frame [bytes] 1024

Table 2. Simulation parameters for ARQ and HARQ

The maximum number of HARQ retransmissions is set to 2 and 4. Both types of HARQ,Chase Combining (CC) and Incremental Redundancy (IR) are considered in evaluations.The Convolutional Turbo Code (CTC) is considered in evaluation if IR HARQ is performed.

4. Results

The results are separated into several groups according to the number of hops (left-hand

and right-hand figures corresponds to one and three hops respectively), HARQ Type (CC

HARQ in Fig. 5 - Fig. 10 and IR-CTC in Fig. 11 - Fig. 16), and maximum number of HARQ

retransmissions for higher clarity. The figures are grouped into set of six figures with the

same HARQ Type, with the same maximum number of retransmissions, and further, withvarying number of hops, ARQ_Block_Size, and PDU Size. The results are presented in form

of figures showing the overhead generated due to ACK/NACK by HARQ and ARQ for

2000 continuously transmitted frames. The expressed overhead is normalized to the

overhead generated by conventional IEEE802.16e ARQ (in figures noted as Conv. ARQ) for

error free channel, using Selective ACK (in figures marked as SACK) together with HARQ

while no interaction between both is considered. The cumulative ACK (CACK) is also taken

into account in figures. All figures also depict results for both techniques while interaction is

not enabled (without interaction - in figures denoted w/o int.) and while the interaction is

enabled (with interaction - in figures noted as w int.). The overhead for the same cases is


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presented also if HARQ and innovative ARQ scheme proposed in (Becvar & Bestak, 2011)

(in figuresARQ PIII) are simultaneously utilized.

As can be observed from Fig. 5 - Fig. 16, the scenario where ARQ and HARQ interact

outperforms all other scenarios. Additional minor improvement is achieved by usinginnovative ARQ instead of conventional ARQ. However, this improvement is noticeable

only as long as ARQ_Block_Size is low (e.g., 16 bytes), PDU Size is higher (e.g., 16 blocks)

and mutual interaction of ARQ and HARQ is considered.

(a) (b)

Fig. 5. ARQ & HARQ Overhead vs.BLER for ARQ_Block_Size = 16 B, PDU Size = 1, Size of

user data = 1024 B/frame, 4 HARQ retrans., HARQ Type: CC, 1 hop (a) / 3 hops (b)

(a) (b)

Fig. 6. ARQ & HARQ Overhead vs. BLER for ARQ_Block_Size = 1024 B, PDU Size = 1, Sizeof user data = 1024 B/frame, 4 HARQ retrans., HARQ Type: CC, 1 hop (a) / 3 hops (b)


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While no interaction between HARQ and ARQ entities is enabled, the difference between

conventional innovative ARQ is more significant. The reduction of overhead is more

appreciable for lower number of hops or higher ARQ_Block_Size. The improvement

achieved by innovative ARQ in comparison to scenario using conventional ARQ without

interaction is due the fact that the ARQ PIII generates lower overhead while the packets aredelivered without errors.

The first group of figures shows the results of CC HARQ for maximum four HARQ

retransmissions.

(a) (b)

Fig. 7. ARQ & HARQ Overhead vs. BLER for ARQ_Block_Size = 16 B, PDU Size = 16, Size of

user data = 1024 B/frame, 4 HARQ retrans., HARQ Type: CC, 1 hop (a) / 3 hops (b)

The next group of figures depicts the results of CC HARQ for maximum two HARQ

retransmissions.

(a) (b)

Fig. 8. ARQ & HARQ Overhead vs. BLER for ARQ_Block_Size = 16 B, PDU Size = 1, Size ofuser data = 1024 B/frame, 2 HARQ retrans., HARQ Type: CC, 1 hop (a) / 3 hops (b)


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The following group of figures represents the results of IR_CTC HARQ for maximum four

HARQ retransmissions.

(a) (b)


(a) (b)



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(a) (b)

Fig. 11. ARQ & HARQ Overhead vs. BLER for ARQ_Block_Size = 16 B, PDU Size = 1, Size ofuser data = 1024 B/frame, 4 HARQ retrans., HARQ Type: IR, 1 hop (a) / 3 hops (b)

(a) (b)

Fig. 12. ARQ & HARQ Overhead vs. BLER for ARQ_Block_Size = 1024 B, PDU Size = 1, Sizeof user data = 1024 B/frame, 4 HARQ retrans., HARQ Type: IR, 1 hop (a) / 3 hops (b)


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(a) (b)


The last group of figures represents the results of IR_CTC HARQ for maximum two HARQretransmissions.

(a) (b)

Fig. 14. ARQ & HARQ Overhead vs. BLER for ARQ_Block_Size = 16 B, PDU Size = 1, Size ofuser data = 1024 B/frame, 2 HARQ retrans., HARQ Type: IR, 1 hop (a) / 3 hops (b)


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(a) (b)


(a) (b)


The impact of individual parameters observed from the previous figures on the efficiency ofoverhead reduction can be summarized into the following concluding remars:

- Number of hops: the efficiency of innovative ARQ scheme is decreasing with highernumber of hops if no interaction is considered; however the reduction of the overheadis not influenced by a number of hops if interaction is enabled.

- ARQ_Block_Size: the more significant reduction of overhead is achieved by utilizationof innovative ARQ with HARQ even without interaction for higher ARQ_Block_Size;additional reduction of overhead by increase of this parameter is enabled by theinteraction for both conventional ARQ as well as for innovative ARQ; the level of thisadditional reduction is getting higher with BLER since the higher BLER increases theamount of packets not corrected by HARQ.

- PDU Size: the overall overhead of ARQ and HARQ rises considerably with PDU size asmore blocks have to be corrected by ARQ since the probability that certain part of PDU


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is delivered with errors is increasing as well; however influence of the level of overheadreduction by this parameter is only minor.

- Maximum number of retransmissions: the impact of a number of retransmissions onthe overhead is negligible in most of scenarios; nevertheless the noticeable overhead

reduction if four retransmissions occur is achieved only for low ARQ_Block_Sizetogether with high PDU Size when interaction is enabled; the reason is that number ofuncorrected errors by two retransmissions do not differ to much from fourretransmissions. Hence the ARQ overhead in similar for both cases.

- Type of HARQ (CC vs. IR): the IR slightly outperforms CC, however the difference inoverall overhead between both HARQ types is also negligible with exception ofscenarios with low ARQ_Block_Size, high PDU_Size and enabled interaction. Thereason for this conclusion is the same as explained in the previous bullet.

5. Conclusions

The chapter investigates the efficiency of ARQ and HARQ mechanism used in WiMAX. Theconventional ARQ, innovative ARQ, and HARQ are described. In addition, theircooperation is contemplated for stand alone operation of both ARQ and HARQ as well asfor cooperation between both.

The results demonstrate that if the HARQ and ARQ are enabled and no mutual interactionbetween both entities is considered, the difference between conventional ARQ andinnovative ARQ is significant. The exact level of overhead reduction depends heavily on thesetting of the ARQ and HARQ parameters. The local interaction between ARQ and HARQenables additional reduction of the overhead. If interaction is considered, the significantimprovement by using innovative ARQ instead of conventional ARQ is achieved only while

ARQ_Block_Size is low and PDU Size is high.

6. Acknowledgment

This work has been performed in the framework of the FP7 project ROCKET IST-215282STP, which is funded by the European Community. The Authors would like to acknowledgethe contributions of their colleagues from ROCKET Consortium (http://www.ict-rocket.eu).

7. References

Becvar, Z. & Bestak, R. (2011). Overhead of ARQ mechanism in IEEE 802.16 networks.

Telecommunication Systems, Vol.46, No.4, (March 2010), pp. 353-367, ISSN 1018-4864

Hoymann, C. (2005). Analysis and performance evaluation of the OFDM-based metropolitanarea network IEEE 802.16. Computer Networks, Vol.49, No.3, pp. 341-363, ISSN 1389-1286

IEEE 802.16e. (2006). Air Interface for Fixed and Mobile Broadband Wireless AccessSystems: Amendment for Physical and Medium Access Control Layers forCombined Fixed and Mobile Operation in Licensed Bands. Standard IEEE

IEEE 802.16j. (2009). Air Interface for Broadband Wireless Access Systems, Amendment 1:Multihop Relay Specification. Standard IEEE


18/18


Kang, M. S. & Jang, J. (2006). Performance evaluation of IEEE 802.16d ARQ algorithms withNS-2 simulator, Proceeding of Asia-Pacific Conference on Communications. Busan,Republic of Korea, 2006

Lee, B.G. & Choi,S. (2008). Broadband Wireless Access and Local Networks: Mobile WiMAX and

WiFi. USA: Artech HouseMaheshwari, S., Boariu, A. & Bacciccola, A. (2008). ARQ/HARQ inter-working to reduce

the ARQ feedback overhead. Contribution to IEEE 802.16m No. C802.16m-08/1142

Martikainen, H.; Sayenko, A.; Alanen, O. & Tykhomyrov, V. (2008). Optimal MAC PDU sizein IEEE 802.16, Proceeding of 4th International Telecommunication Networking Workshopon QoS in Multiservice IP Networks, pp. 66-71

Nuaymi, L. (2007). WiMAX: Technology for Broadband Wireless Access. West Sussex:Wiley&son Ltd.

Provvedi, L.; Rattray, C.; Hofmann, J. & Parolari, S. (2004). Provision of MBMS over theGERAN: technical solutions and performance. Proceeding of Fifth IEEE International

Conference on 3G Mobile Communication Technologies, pp. 494- 498, 2004.Sambale, K.; Becvar, Z. & Ulvan, A. (2008). Identification of the MAC/PHY key

reconfiguration parameters, ICT ROCKET project milestone 5M2, ICT-215282STP

Sayenko, A.; Martikainen, H. & Puchko, A. (2008). Performance comparison of HARQ andARQ mechanisms in IEEE 802.16 networks, Proceeding of International symposiumon Modeling, analysis and simulation of wireless and mobile systems . Vancouver,Canada

Sengupta, S.; Chatterjee, M. & Ganguly, S. (2008). Improving Quality of VoIP Streams overWiMax. IEEE Transactions on Computers, Vol.57, No.2, 145-156

Sengupta, S.; Chatterjee, M.; Ganguly, S. & Izmailov, R. (2005). Exploiting MAC Flexibilityin WiMAX for Media Streaming, Proceedings of World of Wireless Mobile andMultimedia Networks, pp. 338-343, Taormina - Giardini Naxos, Italy, 13-16 June2005

Tykhomyrov, V.; Sayenko, A.; Martikainen, H.; Alanen, O. & Hmlinen, T. (2007).Performance Evaluation of the IEEE 802.16 ARQ Mechanism, Proceeding ofNextGeneration Teletraffic and Wired/Wireless Advanced Networking, pp. 148-161

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